1. Field of the Invention
The present invention relates to an environmental control apparatus for aircraft and, more particularly, to a small-size and light-weight environmental control apparatus for aircraft-external stores. More particularly, the present invention relates to a small-size and light-weight environmental control apparatus for aircraft-external stores which is capable of properly maintaining temperature of the camera unit and the electronic unit of a high-resolution image information acquisition apparatus in an operating environment, such as high-speed movement or an abrupt change of an altitude in a high resolution image information acquisition process.
2. Background of the Related Art
Aircraft is equipped with an image information acquisition apparatus for obtaining an interest target image and transmitting the interest target image to the ground in real time.
The image information acquisition apparatus is equipped with a camera unit and an electronic unit and mounted on the pod of outside the body of the aircraft. The aircraft flies at a high speed at an altitude of about 14 km from a sea level and experiences a thermal load in the pod and an abrupt change of temperature due to the altitude. The change of an environment also affects the image information acquisition apparatus.
In other words, in order to obtain an image having the best quality while not deteriorating the image in a process of the electronic unit processing signals when the camera unit captures the image, there is a need for a temperature control function using thermal components, such as a device for securing cooling air or heating air from the environmental control apparatus, a cooling cycle system, a temperature sensor, a pressure sensor, an air circulation fan, and a heater so that high temperature due to generation of heat in the camera unit and the electronic unit can be cooled and temperature can remain within a specific range using a heating apparatus for a peripheral low temperature environment at a high altitude.
The environmental control apparatus is included in the aircraft in order to control the environment of the image information acquisition apparatus as described above. The environment control apparatus is placed in one section of the pod and is configured to control temperature so that a change of temperature according to a flying condition and an operating state can be minimized and the image information acquisition apparatus can obtain high-resolution image information.
The environmental control apparatus used for the image information acquisition apparatus of aircraft may be chiefly divided into an air cycle cooling apparatus and a steam cycle cooling apparatus.
The air cycle cooling apparatus adopts a method of supplying cooled air obtained by expanding air of high temperature and high pressure, extracted from the compressor of the main engine of the aircraft, into a turbine or a method using a reverse Brayton air cycle cooling apparatus for supplying air of low pressure and low temperature by expanding ram air, introduced into a ram air scoop by the flying speed of the aircraft, into the turbine. The steam cycle cooling apparatus typically obtains air of low temperature by using a change of the phase of a refrigerant in a similar way to a common air-conditioner.
The air cycle cooling apparatus is less sensitive to leakage on a cooling loop and it enables high efficiency cooling during high-speed flying, has a small size and light weight for supplied electric power, and facilitates maintenance and management because cooling and heating devices can be modulated. However, the air cycle cooling apparatus has low efficiency and requires addition electric power suitable for an aircraft speed condition in the ground, and may not obtain constant air temperature because cooling air temperature is greatly changed depending on a flying condition.
The steam cycle cooling apparatus has high efficiency, a low price, and a cooling ability as if it is flying irrespective of the speed of the aircraft even on the ground. However, the steam cycle cooling apparatus is relatively heavier for supply power than an air cycle and sensitive to leakage on the cooling loop, and it has limitations in terms of an operation in a high-speed flying environment and requires regular and consistent maintenance and management.
A condition that the air cycle cooling apparatus is installed and used in the pod includes a case where a high-speed flying condition of the speed of sound or higher or weight and a size are major design factors. A condition that the steam cycle cooling apparatus is used includes a case where cooling requirements, such as when aircraft flies even on the ground, are included, a case where supply power is limited, and a case where a low temperature cooling environment is necessary. When peripheral temperature is low temperature, an additional heating apparatus including a heater and a ventilation fan controls temperature of the image information acquisition apparatus by supplying heating air.
A conventional environmental control apparatus is shown in FIGS. 5 to 7.
FIG. 5 is a schematic diagram of a conventional air cycle cooling apparatus. An air cycle machine 190 for converting air of high temperature and high pressure into cooled air is indicated by a dotted line. Temperature of bleed air 120 that has been controlled by a main engine 100 and extracted by a bleed air controller 110 drops through a primary heat exchanger 130. The bleed air 120 is further compressed through the compressor 140 of the air cycle machine 190. Next, temperature of the bleed air 120 further drops through a secondary heat exchanger 150. The bleed air 120 becomes cooling air in a process in which the air is expanded through a turbine 160. At this time, the temperature of the bleed air is 20 degrees below zero to 70 degrees below zero, and the bleed air cannot be inputted to the pod to be directly cooled. Accordingly, the bleed air is mixed with air of high temperature the amount of supply of which is controlled depending on the degree of opening of a bypass valve 170 so that it becomes air 180 of proper temperature.
FIG. 6 shows a conventional reverse Brayton air cycle cooling apparatus. Air obtained by the flying speed of aircraft and inputted to a ram air inlet port is expanded by the turbine 200 of an air cycle machine, thereby obtaining air of low temperature. Next, the air of low temperature is supplied to a camera unit and an electronic unit via a heat exchanger 220 so that the air absorbs heat and is then externally discharged. In order to prevent the aircraft from overspeeding because the ram air of an excessive pressure is inputted to the air cycle machine when the aircraft flies at the speed of sound, a valve 210 is placed on the turbine inlet side so that the ram air is bypassed. A compressor 230 is driven by expansion motive power of the turbine 200, and the motive power generated from the turbine 200 is increased according to an increase in the rate of available expansion and in the temperature of supplied air. In case of the air cycle machine, however, the driving power of the compressor is small because the rate of available expansion and the supplied air temperature are in a low level. Accordingly, the compressor less contributes to cooling performance than the turbine does because the compressor does not have a high compression ratio.
FIG. 7 shows a conventional cooling cycle and heating system in which the air cycle cooling apparatus and the steam cycle cooling apparatus are mixed according to another example. The cooling and heating system includes a condenser air flow loop 326 using introduced ram air 325 in order to maintain internal temperature of the camera unit and the electronic unit 324 to some degree, a cooling cycle flow loop 327 for circulating a cooling cycle refrigerant, a heating system for raising temperature using a heater 336, and an air circulation loop 328.
The cooling and heating system chiefly includes a ram air scoop 329 (i.e., a ram air inlet port), a ram air turbine 330 for driving a compressor 332, a louver 331 for controlling the amount of air inflow, the compressor 332, an evaporator 333, an expansion valve 340, a ground fan 335, the heater 336, a condenser 339, and a fan 337 for ventilating air, outputted from the evaporator 333, to a camera unit and the electronic unit 324.
In a typical cooling cycle, the compressor, the evaporator, the condenser, and the expansion valve are sequentially connected together by a pipe filled with a refrigerant, thus forming a closed circuit.
An evaporation process is performed by the evaporator. In this process, a refrigerant of a wet gas state having a low temperature and pressure absorbs heat from air while changing into a refrigerant of an overheated steam state having a low temperature and pressure. A compression process is performed by the compressor. In this process, the refrigerant of a wet gas state having a low temperature and pressure is changed into a refrigerant of an overheated steam state having a high temperature and pressure so that external air of normal temperature and the refrigerant within the condenser are subject to a smooth thermal exchange.
A condensation process is performed by the condenser. In this process, the refrigerant of an overheated steam state having a high temperature and pressure is changed into a refrigerant of a liquid state having middle temperature and a high pressure so that a refrigerant of a gas state and the refrigerant of a liquid state can coexist. In an expansion process, the refrigerant of a liquid state having middle temperature and a high pressure is changed into a refrigerant of a wet gas state having a low temperature and pressure so that the refrigerant can be easily evaporated. Accordingly, the refrigerant is repeatedly changed from the liquid state, to the gas state, to the liquid state, thereby forming a cooling cycle.
The construction of the cooling and heating system is described in more detail below.
The ram air turbine 330 is used to obtain motive power necessary to drive the compressor 332 in a common cooling cycle.
The evaporator 333 functions to convert air, heated by a thermal load in the camera unit and the electronic unit 324, into cool air 338 cooled by a thermal exchange with the refrigerant circulated in the cooling cycle or hot air 338 heated by the heater 336. The evaporator 333 includes one heat exchanger, the fan 337 for ventilating air, the heater 336, and so on.
The condenser 339 functions to convert overheated steam, compressed by the compressor 332 in the cooling cycle, into saturated steam by cooling the overheated steam. The condenser 339 is a kind of a heat exchanger.
The expansion valve 334 functions to expand a refrigerant of a liquid state having a high temperature and pressure, transferred from the condenser 339, to a 2-phase flow refrigerant of a liquid state and a gas state having a low temperature and pressure by throttling the refrigerant of a liquid state and then sends the 2-phase flow refrigerant to the evaporator 333.
The heater 336 functions to raise internal temperature of the camera unit and the electronic unit 324 when the internal temperature of the camera unit and the electronic unit 324 is lower than a proper temperature. The fan 337 ventilates air, and the heater 336 supplies the hot air 338 to the camera unit and the electronic unit 324.
Although not shown, the cooling and heating system may include other constituent elements, such as a flow rate pump and a dry valve.
Temperature is chiefly controlled through the three kinds of loop and heating, including the cooling cycle flow loop 326, the condenser air flow loop 327, the heating system by the heater 336, and the air circulation loop 328.
In the condenser air flow loop 327, the introduced ram air 325 rotates the ram air turbine 330, and thus the ram air turbine 330 drives the compressor 332 coupled to the shaft of the ram air turbine. The ram air turbine 330 uses air introduced into the ram air scoop 329 when the aircraft flies and uses high-speed air obtained by operating the ground fan 335 on the ground.
The cooling cycle flow loop 326 includes the compressor 332, the evaporator 333, the expansion valve 334, and the condenser 339 which form a typical cooling cycle. The refrigerant circulates through a pipe line coupled to the constituent elements. A thermal exchange of the refrigerant in the cooling cycle with the air circulation loop 328 in the evaporator 333 functions to control temperature of the hot air 338 heated by a thermal load of the camera unit and the electronic unit 324 so that the hot air 338 becomes cool air. A thermal exchange in the condenser 339 may use the ram air 325 or ventilation air of the ground fan 335. Here, the speed of revolution of the ram air turbine 330 is differently controlled depending on cooling performance of a necessary cooling cycle. The speed of revolution of the ram air turbine 330 is adjusted by an angle of the louver 331 which is placed between the ram air scoop 329 to which external air is inputted and the ground fan 335.
In the air circulation loop 328, the fan 337 lowers or raises the internal temperature of the camera unit and the electronic unit 324 by ventilating and circulating the cooling air of the evaporator 333 or the heating air of the heater 336 to and through the camera unit and the electronic unit 324 so that the internal temperature of the camera unit and the electronic unit 324 maintains a proper temperature.
The condenser air flow loop 326 is described in more detail. Air first entered via the ram air scoop 329 is theoretically compressed in an isoentropic process, and it reaches the front of the ram air turbine 330. The air according to a complete isoentropic process, however does not reach the ram air turbine 330 owing to flow friction between the ram air scoop 329 and the air. Furthermore, as a task is performed in the ram air turbine 330, the temperature and pressure of the air are lowered. The air may be theoretically subject to the isoentropic process, but the air subject to an irreversible process in which entropy is increased reaches the condenser 339. At this time, the air introduced into the condenser 339 is slightly heated and externally discharged through an air outlet. In this case, temperature of the air is theoretically lowered through the isoentropic process and discharged along with air, but the air is subject to a thermodynamic cycle according to the irreversible process. Accordingly, a main object of the condenser air flow loop 326 is to drive the compressor 332 of the cooling cycle flow loop 327 coupled to the ram air turbine 330 of the cooling cycle flow loop 327.
The louver 331 is used to properly adjust the speed of the ram air turbine 330 by controlling the amount of air flow. The louver 331 is driven by an electric motor, and it functions to prevent the overspeeding of the ram air turbine 330 by limiting the speed of revolution of the ram air turbine 330. Sensors are mounted on the louver, and they provide angle and position information to a controller. The louver 331 uses air introduced via the ram air scoop 329 when it is mounted on aircraft and operated and utilizes high-speed air by driving the ground fan 335 in a ground operation condition.
Typically, in order to maximize the cooling efficiency of the cooling cycle flow loop 327, the number of rotations of the compressor, the degree of opening of the electronic expansion valve, and the number of rotations of the fan of each of the evaporator and the condenser need to be controlled. From among them, a method of controlling the capacity of the compressor by varying the number of rotations of the compressor or a method of controlling the degree of superheat of the evaporator by controlling the degree of opening of the electronic expansion valve is used. Furthermore, control for preventing the overheating of the compressor, control of evaporation temperature, control of the flow rate of the refrigerant according to a thermal load of the evaporator, etc. may be performed.
The compressor typically includes a reciprocating compressor, a rotary compressor, a scroll compressor, a screw compressor, and a centrifugal compressor. The operating state of the compressor chiefly depends on condensation temperature, evaporation temperature, and the degree of superheat of a suction gas. The performance of the compressor is determined by three kinds of performance values; volume efficiency, compression efficiency, and machine efficiency. In most compressors, the cooling capacity is set to be greater than a capacity at the time of a standard operation. Accordingly, when the compressor is actually operated, the cooling capacity of the compressor must be controlled. In the compressor, technology for varying the cooling capacity includes control of the speed of revolution, control of a suction volume, loading control, and so on. A method of controlling the cooling capacity includes a method of controlling the cooling capacity by mechanically loading and unloading a fixing scroll and a revolution scroll through control of a solenoid valve according to a pulse width modulation scheme externally, a method of controlling the cooling capacity so that the cooling ability of the compressor is changed in multiple stages based on the opening and shutting of a bypass flow path by constructing a bypass refrigerant valve between upper and lower cylinders, a method of controlling the cooling capacity by varying the speed of revolution of the compressor using an inverter, etc.
From among the control elements of the steam cycle cooling system, control of the degree of superheat of the evaporator has a great effect on the efficiency and stability of the cooling system. That is, when the capacities of the compressor and the evaporator are controlled, the entire system becomes unstable and the compressor may be damaged because the degree of superheat of the evaporator abruptly changes according to a change of the capacities. A capillary tube and a thermostatic expansion valve for controlling the degree of superheat of the evaporator outlet are used.
In the conventional technology, however, the air cycle cooling apparatus of FIG. 5 is inappropriate for an image information acquisition apparatus mounted outside aircraft in the form of a pod. This is because it is difficult for the image information acquisition apparatus mounted outside the aircraft to receive the bleed air 120 from the compressor of the main engine 110 of the aircraft.
The air cycle cooling apparatus of FIG. 6 may have a small size and light weight, but has disadvantages in that it is operated only during flying and it must include an additional cooling air supply apparatus on the ground.
The environmental control apparatus of FIG. 7 in which the air cycle cooling apparatus and the steam cycle cooling apparatus are mixed enables cooling and heating without limitations on the ground or during flying, but is disadvantageous in that a cooling load cannot be rapidly handled, it is difficult to control temperature, and the environmental control apparatus may not be applied to a system having a limited weight because it is too bulky and heavy.
In the conventional compressor, a reciprocating compressor has heavy weight and great vibration and has a problem in low efficiency.
In a capillary tube method initially applied to the expansion valve for controlling the cooling capacity in the conventional cooling cycle, since the section of a flow path is fixed, a change of a cooling load is determined by a state of the degree of supercooling of a refrigerant at the condenser outlet according to a change of the number of rotations of the compressor.
In the refrigerant circulation method, it is difficult to rapidly control the degree of superheat of the evaporator within a necessary range. A Thermostatic Expansion Valve TXV includes a sensing bulb configured to have a refrigerant gas sealed therein and attached to the pipe on the outlet side of the evaporator. The refrigerant gas of the sensing bulb is expanded or contracted by the saturation pressure of the refrigerant within the evaporator and a temperature difference of an exit refrigerant, so that diaphragm coupled to a poppet valve controls the degree of opening of the valve for temperature controlling.
A difference between the steam temperature of a refrigerant and the saturation temperature of the refrigerant is called the degree of superheat. This value can be detected by measuring a difference between an actual temperature of the sensing bulb and a saturation temperature corresponding to the suction pressure of the refrigerant which flows within a pipe line at a position where the sensing bulb is placed. The Thermostatic Expansion Valve TXV functions to prevent the refrigerant from returning back to the compressor, to make almost the entire surface of the evaporator effectively utilized, to make the amount of flow of the refrigerant identical with the amount of the refrigerant that may be evaporated in the evaporator, and to maintain the degree of superheat of the refrigerant within a set range although a thermal load of the evaporator is changed, by controlling the degree of superheat.
However, the pressure of the refrigerant sealed in the sensing bulb may have a detection error owing to a disturbance due to a change of peripheral temperature, and a limit to the deformation of the diaphragm may lead to a delayed forced transfer of the refrigerant when quick cooling is required. Imbalance in the flow control of the refrigerant having a transient state results in a phenomenon in which the refrigerant gas is periodically generated within the evaporator, thus having an adverse effect on the stability of the system. Furthermore, it is difficult to rapidly control the flow against the driving of the inverter of the compressor and a change of a thermal load of the evaporator, and efficient control of the cooling system is difficult.
The conventional cooling system has a very great change in the amount of the refrigerant which is circulated within the cycle because it is operated within a wide temperature range. Accordingly, the conventional cooling system includes an additional liquid receiver for storing an excessive refrigerant according to an operating condition. The additional liquid receiver temporarily stores a refrigerant solution condensed by the condenser and transfers only the amount of the refrigerant, corresponding to an amount consumed within the evaporator, to the expansion valve.
The conventional heating technology is disadvantageous in that the construction is complicated, the heater (i.e., a heating component) is bulky, and an installation space is large because the heating apparatus different from the environmental control apparatus and the control apparatus for controlling the heating apparatus are installed in the camera unit or the electronic unit.